Various types of cooking apparatuses having a heating chamber for containing and heating food are manufactured currently. For example, one of those types of apparatuses includes a magnetron or similar device for generating microwaves to cook food through the dielectric heating method using those microwaves, and another type uses hot air for cooking food. Also, in some of the apparatuses, food is cooked by a combination of these two methods. Further, some apparatuses are known for supplying steam in the heating chamber while heating food with one of the above-described methods. Such types of apparatuses are disclosed in Japanese Examined Patent Publication No. S59-22132, Japanese Unexamined Patent Publication No. H8-49854, and Japanese Unexamined Patent Publication No. H9-4849, for example.
FIG. 11 shows the structure of a steam oven, a conventional cooking apparatus utilizing hot air. The steam oven has a heating chamber 91 provided with a tray 92 inside for mounting an object to be heated. A pot-like enclosure 93 is attached to the back wall of the heating chamber 91, and a fan motor 94 is disposed behind the enclosure 93. The rotation shaft 95 of the fan motor 94 penetrates the wall of the enclosure 93, and a fan 96 is fixed to the end of the rotation shaft 95. A sheathed heater 97 connected to a power circuit (not shown) is provided to surround the fan 96 concentrically.
During a heating process, the fan 96 is rotated for drawing air from the center of the front face and propelling the air toward the circumference. Meanwhile, electric power is supplied to the heater 97, and the propelled air is heated when it contacts the heater 97. The hot air is sent back to the heating chamber 91.
Also, the heating chamber 91 is equipped with a vaporizer 98 having a heater 981 and a tank 982. During the heating process, electric power is supplied also to the heater 981 for vaporizing water in the tank 982 to steam. The steam is supplied into the heating chamber 91 not only for preventing food from being dried by the hot air, but also for improving the heating efficiency. That is, when the heating is carried out with the steam (superheated steam, preferably) being supplied, the steam gives a substantial amount of heat to the food when it contacts the food, so that the time required for cooking becomes shorter than that required in the case where no steam is supplied.
In the above-described steam oven, however, the heat exchanging effectiveness is not high, because the heater 97 is covered with a heat-resistant insulating material and it is structurally difficult to enlarge the surface area of the heater 97. Therefore, if an excessive amount of power is supplied to the heater 97, the temperature of the inside of the heater 97 temporarily becomes abnormally high, which may cause a fault or other damage. Accordingly, the maximum power supply to the heater 97 is limited, which prevents the food from being heated rapidly.
For addressing the above-described problems, the applicant proposed a novel cooking apparatus disclosed in Japanese Unexamined Patent Publication No. H10-255963. The apparatus has a heating chamber for containing an object to be heated, and a cylindrical cup-shaped enclosure made of an insulating material attached to the back wall of the heating chamber. In the enclosure, a fan is provided for drawing air from the center and propelling the air toward the circumference, and a cylindrical heating element is disposed to surround the fan concentrically. A coil is wound around the outside of the cylindrical side wall of the enclosure. The coil is connected to a power supply unit for supplying high-frequency electric power to the coil so that the heating element is inductively heated. In this apparatus, when a high-frequency current is supplied from the power supply unit to the coil, a magnetic flux is generated by the coil. The magnetic flux passes through the cylindrical heating element, whereby an electric current is induced in the heating element circumferentially. Here, the heating element is heated by the Joule heat generated by the induced electric current. Thus, the air propelled toward the heating element is heated.
In this cooking apparatus, a large amount of electric power can be supplied to the coil because the coil itself does not generate heat. Also, the heating efficiency is very high, because the coil and the heating element are disposed adequately close to each other with the cylindrical wall of the enclosure inbetween. Thus, by raising the temperature rapidly, the user can heat food in an adequately short time without damaging its taste or flavor.
In such a cooking apparatus utilizing induction heating, it is preferable to increase the number of loops of the coil to improve the heating efficiency. The number of loops of the coil can be increased by making the cylindrical enclosure longer, but this makes the induction heating unit (consisting mainly of the enclosure, the fan, the heating element and the coil) larger. Taking this problem into account, the applicant further filed Japanese Patent Application No. H9-285996, proposing a cooking apparatus having a shortened induction heating unit. In this apparatus, a heating element in the form of a flat ring is disposed to surround the fan concentrically, and a spiral coil in the form of a flat disc centering on the rotation shaft of the fan is disposed behind the heating element. By such a construction, the number of loops of the coil can be increased without making the induction heating unit larger (or thicker).
In the above-described apparatus, however, it is impossible to increase the diameter of the fan, because it is necessary to keep an adequate space open for the heating element around the fan. Therefore, a strong air flow (or strong wind pressure) cannot be generated when the speed of the fan is low.
For addressing the above-described problems, the present invention proposes a first induction heater applicable to an induction heating unit of a cooking apparatus, which is constructed so that the diameter of the fan can be increased without making the induction heating unit thick or large.
Currently, the most widely used cooking apparatus utilizing induction heating is a domestic induction heater having a top plate and an induction coil placed under the top plate. A pan or pot with food therein is placed on the top plate and is inductively heated. With such an induction heater, it is necessary that the top plate itself does not perform as a load for induction heating, and that the top plate has an adequate heat-resistance. For example, a plate made of insulating material such as ceramic is used as the top plate.
In such an induction heater, an electric current having a high-frequency of about 10 kHz to several tens of kHz, is supplied from a power supply unit to the coil. Here, the current generated by the power supply unit includes also higher harmonic components, and electromagnetic waves including the higher harmonic components are radiated to the outside from the coil, which functions as an antenna.
Most conventional electric or electronic apparatuses are designed to have a shielding means for suppressing leakage of electromagnetic waves to the outside. In the induction heater, however, it is difficult to block the undesired electromagnetic waves effectively because the source of the electromagnetic waves to be shielded is the generator of the magnetic field required for induction heating, which must not be shielded electromagnetically. For addressing this problem, the present invention proposes a second induction heater constructed so that the leakage of high-frequency electromagnetic waves is prevented effectively without decreasing the heating efficiency.